In an RFID system a reader/writer needs a transmitting antenna to convert a voltage created by the reader/writer to an electromagnetic wave that is transmitted as a radio signal to a tag being attached to an object. The reader/writer also needs a receiving antenna to detect a radio signal received from the tag. These two antennas are realized by a pair of antennas being physically separated in close proximity to each other or a single antenna that is used for both transmitting and receiving signals.
However, using the same antenna in different circumstances does not provide the best read rates or the required read zone. Any reflection, diffraction or absorption changes the read zone and anything that propagates a wave can cause signal strength degradation.
If the read zone doesn't cover all the locations in which the user would like to read the tags multiple antennas or multiple pairs of antennas connected to the reader/writer can be used. These antennas or pairs of antennas are not used simultaneously, but are accessed in time sequence to search the read zones available to this particular reader/writer.
However, to configure the reader/writer having multiple antennas or pairs of antennas there is a need to specify which antenna or sequence of antennas should be used and when.
Typically, the reader/writer employs a dipole transmit antenna which is linear in polarization. The RFID tag is usually provided with a linearly polarized antenna such as wire-like metal lines in one direction, a loop or dipole. Exact orientation of the tag antenna is not easy to control. This is problematic especially if the linearly polarized antenna of the reader/writer is orthogonal to the linearly polarized antenna of the tag because there is no electric field along the direction of the propagation axis. This means that no voltage will transfer from the reader/writer to power the microchips of the tag and that communication between the two will be impossible.
Since the orientation of the tag relative to the reader/writer is not easily controlled, omnidirectional antennas being circular in polarization can be used on the tags. Then at least some of the power from the reader/writer with the linearly polarized antenna is usable to power the tag.
However, the use of omnidirectional antennas in tags is critical because their size. There is a need to minimize the size of tags so that they can be attached even to small items. To make the tag small in size means that the size of the antenna has to be reduced. Therefore, the use of small linearly polarized antennas such as loops or dipoles are preferred in tags.